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Abstract

SnO2 nanoparticles were dispersed on graphene nanosheets through a solvothermal approach
using ethylene glycol as the solvent. The uniform distribution of SnO2 nanoparticles on graphene nanosheets has been confirmed by scanning electron microscopy
and transmission electron microscopy. The particle size of SnO2 was determined to be around 5 nm. The as-synthesized SnO2/graphene nanocomposite exhibited an enhanced electrochemical performance in lithium-ion
batteries, compared with bare graphene nanosheets and bare SnO2 nanoparticles. The SnO2/graphene nanocomposite electrode delivered a reversible lithium storage capacity
of 830 mAh g−1 and a stable cyclability up to 100 cycles. The excellent electrochemical properties
of this graphene-supported nanocomposite could be attributed to the insertion of nanoparticles
between graphene nanolayers and the optimized nanoparticles distribution on graphene
nanosheets.

Keywords:

SnO2; Graphene nanosheets; Nanocomposite; Lithium-ion batteries

Background

Graphene has been emerged as a rising star in materials science and as an excellent
candidate for many applications due to its unique two dimensional (2D) nanostructure
[1], outstanding electrical properties [2], and ultrahigh specific surface area [3]. The applications of graphene include gas molecule adsorption [4], quantum dots [5], transistors [6], lithium-ion batteries [7], supercapacitors [8], lithium air batteries [9], and drug delivery [10]. In particular, graphene has attracted worldwide attention for energy storage and
conversion. With the formation of sandwich-like three dimensional (3D) nanostructured
composite materials, the restacking of graphene nanosheets (GNS) can be effectively
prevented and therefore the electrochemical properties of the nanocomposite electrodes
could be significantly improved by using nanocrystallines to insert between layers
of graphene nanosheets [3,11].

SnO2 has been examined as an anode material for lithium-ion batteries with a high theoretical
capacity of 782 mAh g−1[12]. SnO2 forms metal alloys when reacting with lithium, leading to reversible transformations
between lithium tin alloys (LixSn) and tin metal when the lithiation and delithiation proceed. However, the capacity
of bulk SnO2 electrode fades quickly during prolonged cycling [13]. To further improve the electrochemical performance and the cycle life of SnO2 electrodes for long-term cycling, one approach is to synthesize nanosized SnO2 crystals with different morphologies, such as nanowires [14], nanotubes [15], and mesoporous structure [16]. These nanostructured SnO2 materials were reported to deliver greatly enhanced specific capacities with durable
cycling stabilities. In order to mechanically buffer the volume expansion associated
with the charge/discharge processes in the lithium-ion cells, the formation of SnO2/graphene nanocomposites has also been proved to be feasible. Many methods have been
implemented to distribute SnO2 nanocrystals on graphene nanosheets, including in situ chemical preparation [13,17], reassembling process [18], gas–liquid interfacial synthesis [19], as well as hydrothermal and solvothermal methods [20,21].

In this paper, we employ a facile solvothermal technique to disperse SnO2 nanoparticles with a controlled size on graphene nanosheets. The as-prepared SnO2/GNS nanocomposite showed uniform SnO2 nanoparticles distribution and significantly improved electrochemical properties,
compared with bare graphene nanosheets and SnO2 nanoparticles. The solvothermal approach developed in this investigation could be
used for the synthesis of other metal oxide/graphene nanocomposites.

Methods

Graphene oxide (GO) powders were prepared via a chemical approach derived from Hummers'
method [22], according to the previously reported procedure [7]. In a typical synthesis process, 40 mg GO was firstly dispersed in 40 ml ethylene
glycol by ultrasonification for 1 h, followed by the addition of 0.1 mmol SnCl2·2H2O powders. The mixture was vigorously stirred for half an hour, and then transferred
to a 50 ml Teflon lined autoclave, which was sealed and maintained in an oven at 160°C
for 6 h. Afterwards, the black precipitates (SnO2/GNS) were collected, washed with deionized water and ethanol to remove the impurities,
and isolated by vacuum filtration. The product was then dried in a vacuum oven at
60°C, and further sintered at 300°C for 4 h in argon to increase the crystallinity.
For the comparison, bare SnO2 nanoparticles were also synthesized by the same experimental procedure without the
presence of GO in the mixture solution.

The X-ray diffraction (XRD) pattern of the as-synthesized material was measured using
a Siemens D5000 X-ray diffractometer (Siemens Company, Wittelsbacherplatz 2, Munich,
Germany) from 10° to 80° under a scan rate of 1° min−1. The Raman measurement of the SnO2/GNS nanocomposite was conducted on a confocal Micro Raman Spectrometer with LabRAM
HR system (HORIBA Korea Ltd., Pucheon, Kyunggido Korea) using a 632.8 nm He-Ne laser
source. The spectra were recorded in the range of 200 to 2,000 cm−1 with accumulated scans for an enhanced resolution. Field emission scanning electron
microscope (FESEM) observations were performed using a Zeiss Supra 55VP FESEM with
an Oxford energy dispersive spectrometry system (Carl Zeiss Nanotechnology Center,
Oberkochen, Germany). The transmission electron microscopy (TEM) analysis was carried
out using a Jeol 2011 TEM facility (JEOL Ltd., Tokyo, Japan). The graphene (carbon)
content in the composite material was determined by thermogravimetric analysis (TGA)
on a TGA/DTA analyzer (TA Instruments, SDT 2960 module, New Castle, DE, USA) in air
at 10°C min−1 ranging from room temperature to 1,000°C.

CR2032 coin cells were assembled in an argon-filled glove box (Unilab, M Braun Inertgas-Systeme
GmbH, Garching, Germany) in which the levels of moisture and oxygen were controlled
to be less than 0.1 ppm. The electrodes were made by mixing 80 wt% SnO2/GNS active materials, 10 wt% carbon black, and 10 wt% polyvinylidene fluoride binder
in the N-methyl-2-pyrrolidone solvent to form a slurry. Then, the slurry was coated on copper
foil substrate. Lithium foils were used as the negative electrodes. The electrolyte
was 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (1:1). Cyclic voltammetry (CV) tests
were carried out on an electrochemistry workstation (CHI660D, CH Instrument, Inc.,
Austin, TX, USA) at a scan rate of 0.1 mV s−1 vs. Li/Li+ reference electrode in the voltage range of 0.01 to 3 V. Galvanostatic charge/discharge
measurements were conducted on the Neware battery tester (Neware Co.,Ltd., Shenzhen,
China) with a current rate of 0.1 C for 100 cycles. Electrochemical impedance spectroscopy
was performed on the same electrochemistry workstation. The frequency was set in 0.01 Hz–100 kHz
with the amplitude of 5 mV. The charge/discharge performance was also investigated
for bare graphene nanosheets and SnO2 nanoparticles as a comparison.

Results and discussion

Figure 1 shows a schematic diagram of the formation of the SnO2/GNS nanocomposite. Firstly, Sn2+ ions were attracted to GO nanosheets in the ethylene glycol (EG) solution. Then,
the anchored Sn2+ ions were reduced by EG via the following two-step reactions:

(1)

(2)

Figure 1 .Schematic diagram of the formation of SnO2/GNS nanocomposite.

Simultaneously, GO nanosheets were also gradually reduced by EG to form graphene nanosheets.
As EG is a mild reducing agent, the reduction processes take a long time (6 h) to
complete at the high temperature (160°C). Then, the formed Sn nanoparticles were further
oxidized by oxygen to become SnO2 nanoparticles during the cooling period. Consequently, the SnO2/GNS nanocomposite was obtained under the assistance of EG which acted not only as
a dispersing agent but also as a reducing agent.

XRD patterns of the SnO2/GNS nanocomposite and graphene were shown in Figure 2. In Figure 2a, X-ray diffraction lines of well-crystallized SnO2 were indexed to the tetragonal rutile SnO2 phase, which is consistent with the Joint Committee on Powder Diffraction Standards
card 01–0657. The weak peak of graphene is also visible at 42° in the composite, which
matches the diffraction peak (1 0 0) marked on the XRD pattern of bare graphene nanosheets
(as shown in Figure 2b). XRD confirmed the coexisting phases of rutile SnO2 and graphene and the formation of SnO2/GNS nanocomposite.

Raman spectra of the as-prepared SnO2/GNS nanocomposite and bare GNS are shown in Figure 3. It can be seen that both of them show high-intensity D band and G band at around
1,327 and 1,587 cm−1, respectively. The D band is stronger than the G band, and the D/G intensity ratio
increased significantly compared to pristine graphite [23], which confirmed the existence of graphene nanosheets in the composite material.
The D/G intensity ratio of the SnO2/GNS nanocomposite is higher than that of the bare GNS, indicating the decrease of
the sp2 carbon domains when SnO2 nanoparticles were inserted between graphene nanosheets [24]. The inset in Figure 3 displayed magnified Raman spectra in the range of 400 to 900 cm−1 of both SnO2/GNS and bare GNS. Two weak peaks were found at 465 and 620 cm−1 in the Raman spectrum of the SnO2/GNS nanocomposite, which can be assigned to the Eg and A1g active modes of SnO2 crystallines [25]. For the bare GNS, no Raman peak in this range was observed. TGA was employed to
determine the weight composition of the SnO2/GNS nanocomposite (as shown in Figure 4). The dramatic weight loss from 500°C to 630°C is associated with the burning of
graphene in air. SnO2 in the nanocomposite was stable up to 1,000°C. Therefore, the composition of the
SnO2/GNS nanocomposite was calculated to be 36.3 wt% SnO2 and 63.7 wt% graphene.

Figure 3 .Raman spectra of SnO2/GNS nanocomposite and bare GNS from 200 to 2,000 cm−1. The inset shows magnified views of the spectra in the range of 400 to 900 cm−1.

Figure 5 displays FESEM images of the SnO2/GNS nanocomposite. Corrugated graphene nanosheets are well expanded and form flower-like
nanostructure (Figure 5a). A magnified scanning electron microscope (SEM) view further revealed details of
a large, flat graphene nanosheet (Figure 5b). Tiny SnO2 nanoparticles were found anchored on this graphene flake. Figure 6 shows a TEM image of the SnO2/GNS nanocomposite. A large amount of SnO2 nanoparticles were homogeneously distributed on the graphene nanosheets as shown
in Figure 6a. The inset shows the selected area electron diffraction pattern (SAED). The diffraction
rings were indexed as the crystal planes (1 1 0), (1 0 1), (2 0 0), (2 1 1), (2 1
0) of SnO2, which clearly confirms the presence of SnO2 in the nanocomposite material. High resolution TEM (HRTEM) was performed on a few
SnO2 nanoparticles (Figure 6b). SnO2 nanocrystals were densely packed on the surface of graphene nanosheets. Two crystal
planes were indexed to be (1 0 0) and (1 1 0) of SnO2. The particle size of SnO2 was determined to be around 5 nm.

Figure 5 .SEM images of SnO2/GNS nanocomposite: (a) a low magnified image showing flower-like microstructure of
graphene nanosheets and (b) a high magnified image focusing on a large graphene flake.

Figure 7 presents typical CV characteristics related to the lithiation and delithiation processes
of the SnO2/GNS nanocomposite in the lithium-ion cell. A small cathodic peak appears at 0.8 V
in the first cycle which can be attributed to the formation of the solid electrolyte
interphase layer. Another small reduction peak located around 0.06 V could be due
to the reactions between lithium and SnO2 nanoparticles to form LixSn alloys, while the insertion of lithium in graphene nanosheets could be identified
as the reduction peak at 0.01 V. There are three oxidation peaks located around 0.13,
0.55, and 1.3 V, respectively. They correspond to different oxidation reactions during
the charge process. The first anodic peak at 0.13 V represents the lithium extraction
from graphene nanosheet. The 0.55 V oxidation peak can be assigned to the dealloying
of LixSn, showing a reversible process. The third weak oxidation at 1.3 V could be resulted
from the partial transformation of Sn metal to SnO2[26,27]. The high reversibility of the CV curves further confirmed the reversible redox reactions
occurring in the lithium-ion cell between lithium and SnO2/GNS nanocomposite.

Figure 7 .CV curves of the SnO2/GNS nanocomposite electrode at the 1st, 2nd and 5th cycle at a sweep rate of 0.1 mV s−1.

Figure 8 shows the charge/discharge profiles of the SnO2/GNS nanocomposite electrode in different cycles at a current rate of 0.1 C. It can
be seen that the SnO2/GNS electrode delivered a discharge capacity of 1,542 mAh g−1 in the first cycle. From the second cycle, the nanocomposite electrode exhibited
highly reversible charge and discharge capacities. The maximum reversible discharge
capacity of 830 mAh g−1 was achieved in the second discharge cycle. Specific discharge capacities of 588
and 561 mAh g−1 were obtained in the 50th and 100th cycle respectively, which indicates a very stable
cycling performance. Significant improvement on the specific capacities has been achieved.
Figure 9 shows the long-term cycling properties of the SnO2/GNS nanocomposite, bare graphene nanosheets and SnO2 nanoparticles at a 0.1 C current rate. The SnO2/GNS nanocomposite electrode demonstrated the highest reversible capacities and the
best cycling stability. The nanocomposite electrode delivered a discharge capacity
of 1,542 mAh g−1 in the first cycle and maintained stable capacities from the second cycle for 100
cycles with an excellent capacity retention. On the other hand, the bare GNS electrode
showed a large irreversible capacity with lower reversible discharge capacities in
100 cycles. The capacities of SnO2 nanoparticles decrease quickly upon cycling. The retained capacity was less than
30 mAh g−1 in the 100th cycle. Figure 10 demonstrates multiple-step cycling characteristics of the SnO2/GNS nanocomposite electrode at 0.05 to 0.1, 0.2, 0.5, and 1 C and then reversing
back to 0.1 and 0.05 C. The nanocomposite electrode was capable to deliver stable
specific capacities at various current rates and recover substantial capacities without
obvious capacity decline when returning to lower current rates. This indicated a fully
reserved microstructure of the nanocomposite electrode after cycling at higher current
rates.

Figure 8 .Charge/discharge profiles of the SnO2/GNS nanocomposite electrode at a current rate of 0.1 C.

Figure 10 .Multiple-step cycling characteristic of the SnO2/GNS nanocomposite electrode at different current rates.

Figure 11 shows the alternating current (AC) impedance spectra of the SnO2/GNS nanocomposite electrode before cycling, after 5 cycles and after 100 cycles and
the equivalent circuit model (inset). The intercept on the Z' axis at the high frequency region represents the resistance of the electrolyte (Rs), which is 56.2 Ω for a fresh cell. The electrolyte resistance slightly decreased
to 35 Ω after 5 cycles and remained nearly unchanged after 100 cycles (36.2 Ω). The
diameters of the semicircles on the spectra implied the charger transfer resistances
(Rct) at the electrolyte/electrode interface. It should be noted that the initial charge
transfer resistance was 575.9 Ω then gradually decreased to 242.5 Ω (after 5 cycles)
and 95.25 Ω (after 100 cycles) upon prolonged cycling. The significantly decreased
charge transfer resistance could benefit for an enhanced cycle life of the SnO2/GNS nanocomposite electrode. The overall electrochemical performance of the SnO2/GNS nanocomposite was improved as graphene nanosheets supported SnO2 nanoparticles on their layered nanostructure. The inserted SnO2 nanoparticles reduce the stacking degree of graphene nanosheets and also contribute
to the reversible lithium storage. Graphene nanosheets in the nanocomposite not only
accommodate the volume change associated with the reactions between lithium and SnO2 nanoparticles, but also provide electrical conductance for the electrodes. For the
SnO2/GNS nanocomposite prepared by the solvothermal method, the electrochemical properties
were further improved due to the optimized nanoparticle distribution and small particle
size of SnO2. The well-dispersed SnO2 nanoparticles effectively prevent the formation of agglomerates on graphene nanosheets,
which induces an enhanced electrochemical performance.

Conclusions

A facile solvothermal preparation method has been developed to synthesize the SnO2/GNS nanocomposite with a uniform nanoparticle distribution. The as-prepared SnO2/GNS nanocomposite exhibited an improved lithium storage capacity and cycling performance
compared to bare GNS and bare SnO2 nanoparticles. The presence of GNS in the nanocomposite could increase the electrical
conductivity and buffer the volume expansion associated with the lithiation and delithiation
processes, leading to a significantly enhanced electrochemical performance. The solvothermal
approach might be applicable for rapid and effective synthesis of other metal oxide/graphene
nanocomposites.

Abbreviations

GNS,, Graphene nanosheets; GO,, Graphene oxide.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

BW designed and carried out the experimental work, conducted basic characterizations
of the sample, undertook all the electrochemical tests, analyzed all the data, and
wrote the manuscript. DS performed the TEM observations. JP obtained the Raman spectra
of the samples. AH and GW supervised the research work and GW critically revised the
manuscript. All authors read and approved the final manuscript.

Acknowledgment

This work was financially supported by the Australian Research Council (ARC) through
the ARC Discovery Project (DP1093855). We also acknowledge the support from the National
Research Foundation of Korea through the World Class University program (R32-2008-000-20093-0).